The invention relates to a method as in claim 1, as well as to a transmitter and receiver arrangement for implementation of the method as in claim 12.
In wireless information transfer methods, that are well known to the expert from standard reference works, the information signal to be transmitted is modulated upon a high frequency carrier signal in the transmitter and transferred over a transmission path to the receiver, which contains a corresponding demodulator for the recovery of the information signal. A well known modulation method in telecommunications is the angle modulation (as generic term for frequency and phase modulation).
If the information signal to be transmitted is present in digital form as a bit sequence, as is the case in modern mobile radio networks, then the modulation is carried out by variation of the frequency, or phase, or amplitude of the carrier signal, depending on the bit sequence to be transmitted. Various digital modulation methods are known, for example from COUCH, L. W.: Digital and Analog Communication Systems, 4th Edition, Macmillan Publishing Company (1993), among them amplitude-shift keying (ASK: Amplitude Shift Keying), two phase-shift keying (2-PSK: Phase Shift Keying) or two frequency-shift keying (2-FSK: Frequency Shift Keying). Here too a demodulation is carried out in the receiver according to the modulation method employed on the transmitter side, thus effecting a recovery of the digital information signal as a bit sequence in form of consecutive pulses.
The use of several different modulation methods for different messages, or message components, as part of a continuous transmission process is known to the expert, for example from analogue television engineering, where the vestigial side-band amplitude modulation is used for the luminance signal, the frequency modulation for the audio signal, and the IQ modulation for the chrominance signal. Here too, the variation of the carrier parameter s serves only in the imprinting of the information and has no effect on noise of the transmission path.
A method for expansion of emitted tracking pulses on the transmitter side and compression on the receiver side is known from radar technology (xe2x80x9cChirpxe2x80x9d-technique); compare E. Philippow (Publisher.): Taschenbuch der Elektrotechnik, Vol. 4, Systeme der Informationstechnik, Berlin 1985, p. 340,341. Hereby an analogue frequency modulation or a digital phase modulation is applied in the compression, but no imprinting of information takes place. This method serves in the reduction of the expended transmission power, and thus a potential opponent""s ability to detect the signals, while simultaneously maintaining range and accuracy of coverage.
A basic physical problem exists in all communication methods: the quality of the information signal that is recovered on the receiver side decreases with the amount of interference on the transmission path (always present in reality), and thus with the distance between transmitter and receiver. To obtain a desired working distance at a predetermined noise immunity in a communication over a noisy transmission path, a certain transmission power is necessary, which, for example for mobile communications, is in the range of Watts.
On one hand, the required transmitting power has the disadvantage that the energy consumption during the transmitting operation is correspondingly high, which in particular for battery or accumulator battery operated devices, such as mobile telephones, is a problem, due to the rapid depletion of the energy store. On the other hand, the rising, number of communication transmitters caused by the explosive distribution of mobile telephones, the increasing number of providers of radio broadcasts and television programs etc, increases the total impact of electromagnetic radiation on humans (so called xe2x80x9chuman exposurexe2x80x9d). Harm to the human body can not be ruled out, in particular for mobile telephones at the presently customary transmitter power, due to the very low distance of the transmitter to the use""s head.
This invention has the objective to develop a method of the type mentioned at the beginning, and an arrangement for the implementation thereof, which allows a reduction in transmission power and/or and increase in range while maintaining at least equal transmission quality.
This objective is met, starting with a method according to claim 1, by this method""s characterizing features, andxe2x80x94regarding the arrangement for implementing the methodxe2x80x94by the features of claim 12.
The invention includes the principal thought to use two independent modulation methods to imprint the information onto a carrier (information signal modulation) and to achieve extensive suppression of noise on the transmission path, in particular of the thermal or xe2x80x9cwhitexe2x80x9d noise (carrier signal modulation).
The pulses that have been modulated, or are to be modulated, with the information according to a well known method of telecommunications, in the transmitter are subjected to an angle modulation (which here is to be understood as generic term for phase and frequency modulation) with a special characteristic. The angle modulated pulses, showing a predetermined frequency spectrum, are time compressed in the receiver by introducing a frequency dependent delay. Thus an amplitude enhancement results at the receiver output, compared to the amplitude of the transmitted signal, and thus to the noise level. In particular, this pulse compression/amplitude enhancement can be carried out using a dispersive filter. The information signal is recovered from the carrier processed in this manner by demodulation, whereby the demodulation of the information signal occurs with a signal/noise ratio improved by the amplitude enhancement.
The improvement of the signal/noise ratio is dependent on the bandwidth-time-product of the bandwidth used in the angle modulation and the pulse duration, and is especially prominent in poor transmission conditions.
The actual information can be imprinted onto the carrier by pulse modulation techniques, or by carrying out the carrier compression so that it can be evaluated in different ways for different states of the information signal, so that the information is contained in this variation of the angle modulation. Hereby it is important that the modulation of the information has no, or only secondary, influence on the signal delay time.
After the demodulation the available signal is of a quality, which in the state of technology could only be achieved by increased transmitting power or by costly methods for the improvement of reception (such as diversity reception or redundant transmission). A further advantage of this invention""s method lies in the essentially lower potential for interference compared to other transmission paths, because a predetermined signal/noise ratio can be achieved after the pulse compression in the receiver using lower transmitting power. In addition, the lower demands on the transmitting power lead to a reduced human exposure. The disadvantage of this method, a higher required bandwidth, and thus a reduced channel capacity or transfer rate (bit rate) can be accepted for many areas of application, and can be partially eliminated through the selection of a matching pulse modulation method for the modulation of the information (see below).
A special angle modulation time characteristic is used in the variable angle modulation, which corresponds to a xe2x80x9cmodulation characteristic curvexe2x80x9d. Hereby, the modulation characteristic curvexe2x80x94here referred to as modulation characteristicxe2x80x94determines the time behavior of the frequency during the duration of each pulse. When a linearly falling modulation characteristic is used, the frequency of the transmitted signal decreases linearly, during the duration of each pulse, from a value above the carrier frequency to one lying below the carrier frequency. Analogously, a linearly rising characteristic can be used. The filter on the receiver side is matched to the (employed modulation characteristic by a corresponding differential, frequency dependent delay time response (group delay response) in such a way that the signal components of different phase position, generated on the transmitter side are superimposed to a signal nearly coincident in time (approximate xcex4-pulse).
In an advantageous embodiment of the invention the imprinting of the information of the input signal occurs by selecting or varying the modulation characteristic depending on the input signal. If the input signal contains a high-level, then, for example, a modulation characteristic decreasing (most simply linearly) with the signal is used, which leads to a frequency modulated pulse with a frequency decreasing during the pulse duration (xe2x80x9cDown-Chirpxe2x80x9d). In contrast, a (linearly) rising modulation characteristic is used for a low-level of the input signal, which correspondingly yields a pulse with a frequency that rises during the pulse duration (xe2x80x9cUp-Chirpxe2x80x9d).
The filter mean; on the receiver side are matched by an inverse or complementary characteristic. If the angle modulation on the transmitter side is carried out according to a decreasing modulation characteristic, then the frequency of the pulse decreases during the pulse duration, which has as a result that the signal components of higher frequency arrive on the receiver side before the signal components of lower frequency. Thus, the delay time response of the dispersion filter on the receiver side has to compensate for the xe2x80x9cleadxe2x80x9d of the high frequency signal components, so that the spectral signal components of the frequency modulated pulse superpose to form a pulse with increased amplitude at the output of the dispersion filter.
To transmit a higher information content with each pulse, it is possible to use more than two modulation characteristics for the input signal. If, for example, four modulation characteristics are available, then accordingly four different pulses can be transmitted, which corresponds to an information content of 2 bit for each transmitted pulse. By increasing the number of different modulation characteristics, the data transfer rate can be advantageously increased, whereby it must be noted that the technical expense increases at the same time, and the different pulses with a very large number of different modulation characteristics become more difficult to distinguish, which increases the transmission""s susceptibility to errors.
In the previously described variation of the invention, the modulation of pulses is carried out actively for a high-level as well as for a low-level of the digital input signal. This means that frequency modulated pulses are generated for a high-level as well as for a low-level of the input signal, which can be distinguished by the type of frequency variation during the pulse duration. Hereby, the imprinting of the information contained in the input signal occurs through selection or variation of a modulation characteristic, depending on the input signal.
Alternatively, the transmission of the input signal can be carried out actively for only one of two defined levels, while no pulse is generated for the other level. For example, a linearly rising frequency modulated pulse is generated for a high-level of the input signal, while a pause of the pulse""s length is inserted for a low-level. This variation of the invention allows implementing the method using a single modulation characteristic, with low technical expense. In particular, only one dispersion filter is required on the receiver side.
The imprinting of the information contained in the input signal onto the transmitted signal occurs according to a known modulation methods, which are at least approximately orthogonal, for example: a pulse code modulation (PCM), or a differential pulse code modulation (DPCM), or a pulse delta modulation (PDM), preferably using pulse position modulation (PPM), in which the position of the individual frequency modulated pulses is varied relative to a reference pulse, depending on the input signal. Application of the pulse phase- or pulse width modulations is in principle suitable, but potentially requires higher technical expense, or does not match all the advantages of the PPM.
Using the combination of xe2x80x9cchirpxe2x80x9d modulation, for carrier noise suppression, and PPM, for imprinting the information, lends itself in a particularly advantageous manner for utilizing the increase in time resolution on the receiver side, that arises in the pulse compression of pulses with very short rise time, for increasing the transmission rate (with respect to the increased band width), by utilizing the superposition principle in the reception of pulses overlapping in time. Seen in its entirety, this allows for extensive compensation of the original loss of transmission rate. A (small) portion of the transmitting power saved due to the compression is employed for the emitting of the reference pulses needed for the PPM, and potentially for additional encoding pulses in the same channel.
The recovery of the information that is contained in the input signal is effected by a detector, connected after the dispersion filter, that is matched to the modulation method that is employed for imprinting the information, contained in the input signal, on the transmitter side.
If one of several modulation characteristics is selected on the transmitter side, depending on the amplitude of the input signal, preferably a linearly falling modulation characteristic for a high-level and a linearly rising modulation characteristic for a low-level of the input signal, then two options exist for the interpretation in the receiver.
One option consists of providing only one dispersion filter on the receiver side, the differential phase delay, or group delay response, of which is matched to one of the modulation characteristics used on the transmitter side in such a way, that the signal components of the pulse, frequency modulated according to this modulation characteristic, arrive superposed at the output of the dispersion filter, which leads to a pulse compression and increase in amplitude. For a pulse of one of the other modulation characteristics, that is not optimally matched to the delay time response of the dispersion filter on the receiver side, the spectral signal components arrive spread over time at the output of the dispersion filter, and thus, due to the lower pulse compression, with lower amplitude. Thus, in this embodiment the amplitude of the pulse arriving at the output of the dispersion filter depends on the modulation characteristic employed on the transmitter side, and thus on the amplitude of the input signal that was used in the selection of the modulation characteristic. To recover the digital input signal from the output signal of the dispersion filter, an amplitude sensitive detector, potentially executed as amplitude demodulator, is connected after the dispersion filter.
In the other option the frequency modulated pulse is fed to several dispersion filters, connected in parallel, on the receiver side. The frequency depending delay time response of the dispersion filter on the receiver side and the modulation characteristics used on the transmitter side are matched in pairs, in such a way that the signal components of the frequency modulated pulse arrive compressed at the output of exactly one of the dispersion filters, thus leading to an increase in amplitude, while no increase occurs in the output signals of the other dispersion filters, due to the different characteristic. Thus the input signal can be discriminated according to the particular dispersion filter at which an increase in amplitude is present.
Advantageously, the dispersion filters are executed as surface acoustic wave filters (xe2x80x9cSAW filterxe2x80x9d), which can be manufactured with high accuracy and stability. In addition, SAW filters offer the advantage that amplitude response and phase response can be dimensioned independently of each other, which offers the possibility to execute the narrow banded band-pass filter required in each receiver and the dispersion filter as one component.
The generation of the frequency modulated signal in the transmitter can occur in different ways, some of which will be briefly described as examples in the following.
In an advantageous variation of the invention, at first an approximate (quasi-) Dirac pulse is generated and fed to a low-pass filter, the filter characteristic of which possesses a peak shortly before the critical frequency, and thus transforms the delta-pulses into Sinc-pulses, the shape of which is described by the well known Sinc-function, Sinc(x)=sin(x)/x. Subsequently, the Sinc-shaped output signal of the low-pass filter is led to an amplitude modulator that imprints the Sinc-shaped envelope onto a carrier oscillation. If the signal generated in this manner is fed to a dispersive filter, then a frequency modulated pulse appears at the output. Thus in this variation of the invention, at first a dispersion filter on the transmitter side expands the relatively sharp Sinc-pulse into a frequency modulated pulse, which is broadened compared to the Sinc-pulse and has a correspondingly lower amplitude. A compression of the pulse, with a corresponding increase in amplitude, subsequently occurs on the receiver side, also using a dispersion filter. Since one dispersion filter each is used for the expansion of the pulses on the transmitter side, and for the compression on the receiver side, this variation of the invention is advantageously suited for a transceiver operation with alternating transmitting and receiving operation. For this purpose, the transmitter and receiver can contain corresponding identical component modules with one dispersion filter each, that in transmitting operation serve in the generation of the frequency modulated pulse, and in receiving operation help in the compression of the received frequency modulated pulse.
In another variation of the invention, the generation of the frequency modulated pulses; is effected using a PLL (PLL: Phase Locked Loop) and a voltage controlled oscillator (VCO: Voltage Controlled Oscillator). The individual pulses of the input signal, that is present in digital form, hereby are at first converted to saw-tooth shaped pulses in an integrator, whereby the rise direction of the individual pulses depends on the amplitude of the input signal. The signal generated in this manner is then used for triggering the VCO, so that the frequency of the output pulse linearly increases or decreases during the pulse duration, depending on the level of the input signal.
In a further variation of the invention, a digital signal-processing unit generates the frequency modulated pulse in the transmitter, which advantageously allows the implementation of any desired modulation characteristics.
In a variation of the invention, matched transmitter-receiver pairs are produced to implement the complementary transmitter-receiver characteristics, so that no further tuning work is required when the system is put in operation.
In another variation of the invention the receiver is matched to the transmitter before or during the operation, by varying the delay time response of the dispersion filter used on the receiver side. Hereby, the transmitter, as part of a matching process, generates a reference signal, which preferably corresponds to a series of high-levels of the input signal, whereby the modulation characteristic of the frequency modulation carried out on the transmitter side, or the frequency dependent delay time response of the dispersion filter on the receiver side, are varied until an optimum pulse compression or increase in amplitude occurs on the receiver side. This variation is especially advantageous when using a digital signal processor for filtering and processing in the receiver, since such a signal processor allows in simple manner a variation of the frequency dependent delay time response and a corresponding optimization, whereby the optimization procedure can be executed automatically, by computer control.
In a further advantageous embodiment of this variation, the data transfer occurs block by block, whereby the above mentioned matching process is carried out renewed for each block, to be able to dynamically compensate for fluctuations of the dispersion characteristics on the transmission path.
Advantageous further developments of the invention are identified in the secondary claims, or will be described, together with the invention""s preferred embodiment, in more detail in the following.